This invention relates generally to a raster output scanning system for producing a high intensity imaging beam which scans across a movable photoconductive member to record electrostatic latent images thereon, and, more particularly, to an apparatus for providing registration of the beam in the process direction movement of the photoconductive member.
In recent years, laser printers have been increasingly utilized to produce output copies from input video data representing original image information. The printer typically uses a Raster Output Scanner (ROS) to expose the charged portions of the photoconductive member to record an electrostatic latent image thereon. Generally, a ROS has a laser for generating a collimated beam of monochromatic radiation. The laser beam is modulated in conformance with the image information. The modulated beam is reflected through a lens onto a scanning element, typically a rotating polygon having mirrored facets. The light beam is reflected from a facet and thereafter focused to a "spot" on the photosensitive member. The rotation of the polygon causes the spot to scan linearly across the photoconductive member in a fast scan (i.e., line scan) direction. Meanwhile, the photoconductive member is advanced relatively more slowly than the rate of the fast scan in a slow scan (process) direction which is orthogonal to the fast scan direction. In this way, the beam scans the recording medium in a raster scanning pattern. The light beam is intensity-modulated in accordance with art input image serial data stream at a rate such that individual picture elements ("pixels") of the image represented by the data stream are exposed on the photosensitive medium to form a latent image, which is then transferred to an appropriate image receiving medium such as paper. Laser printers may operate in either a single pass or multiple pass system.
In a single pass, process color xerographic system, three ROS stations are positioned adjacent to a photoreceptor surface and selectively energized to create successive image exposures, one for each of the three basic colors. A fourth ROS station may be added if black images are to be created as well. In a multiple pass system, each image area on the photoreceptor surface must make at least three revolutions (passes) relative to the transverse scan line formed by the modulated laser beam generated by a ROS system. With either system, each image must be registered to within a 0.1 mm circle or within a tolerance of .+-.0.05 mm. Each color image must be registered in both the photoreceptor process direction (skew registration) and in the direction parallel to the process direction (referred to as fast scan or transverse registration). Registration in the transverse direction of a single pass ROS system is known in the prior art and a preferred registration technique is disclosed in U.S. Pat. No. 5,237,521 issued on Aug. 17, 1993, assigned to the same assignee as the present invention. Contents of this application are hereby incorporated by reference.
The following references were located during a pre-ex search: U.S. Pat. No. 4,893,135 to Jamzadeh; U.S. Pat. No. 4,837,636 to Daniele et al.; Japanese Patent No. 62-244066 to Ito; Japanese Patent No. 62-266575 to Kaneko and Japanese Patent No. 62-287270 to Mitekura. Japanese Patent No. 62-266575 to Kaneko discloses a method for achieving color registration in a multiple laser scanning system. An image recording signal is outputted, and each laser beam 2a-2d begins scanning after a particular respective set amount of time Na--Nd. Each particular set time Na--Nd for beginning scanning at each imaging station is shortened or extended by an amount proportional to a color shear value (i.e. a misregistration amount) detected by a corresponding optical position detector 12a-12d at each station. Japanese Patent No. 62-244066 to Ito, assigned to Fuji Xerox Co. Ltd., discloses a method for synchronizing multiple deflecting mirrors wherein a crystal oscillator is used to generate a stable speed reference clock. The speed reference clock is supplied in common to scanning speed control circuits 28B, 28C, 28M and 28Y for synchronously controlling deflecting mirror motors 22B, 22C, 22M and 22Y. Japanese Patent No. 62-287270 to Mitekura discloses a method of accurately superimposing plural images on one sheet of recording paper. Laser beam exposure among plural image forming devices is automatically controlled according to speed variations of a photosensitive drum and a transfer belt. U.S. Pat. No. 4,893,135 to Jamzadeh discloses a method of enhancing image registration in laser printers. The enhancement is provided by delaying the feeding of paper into an image transfer station by an amount of time a latent image was delayed in scanning at an exposing station when scanning was to begin. A time difference between a start of page signal and a start of line signal is converted to time delays used to extend or delay a conventional signal issued by a master system controller to feed paper into the transfer region. Xerox Disclosure Journal publication entitled "Pulse-Width Modulation Control of Brushless Motors" (Schweid et al., Vol. 16, No. 3 May/June 1991, p. 153) relates to pulse width modulation control of brushless DC motors. U.S. Pat. No. 4,837,636 to Daniele et al. discloses a photoreceptor having marks placed thereon, used to determine photoreceptor speed and position.
The present invention is directed towards a method and apparatus for registering the color images in the process direction. Errors of the spot position in the process direction can originate from a number of sources including polygon facet and/or motion flaws, changes in the velocity of the photoreceptor, and changes in the spacing, in the process direction, between successive ROS units. The present invention is directed to eliminating all three types of errors. In a first embodiment, a start-of-scan sensor is connected via a feedback loop with each polygon ROS motor. A desired reference frequency, generated as an internal, digital value within a microcontroller, is compared with the start-of-scan (SOS) sensor output within said microcontroller using the technique described in U.S. Pat. No. 5,208,796 issued on May 4, 1993, assigned to the same assignee as the present invention, whose contents are hereby incorporated by reference. The commanded voltage to the motor is adjusted until the frequency of the start-of-scan signal coincides with the desired reference frequency. The same microcontroller is used to control the motion of each of the ROS motor polygon assemblies and thus the use of a common, internal digital value ensures that all ROS units are rotating at the same, identical frequency.
In a second embodiment, the relative frequency error between the motion of the motor polygon assemblies and that of the photoreceptor is further reduced by using encoded signals representing the photoreceptor velocity as the reference signal to be compared with the SOS signals within a microcontroller using the technique described in U.S. Ser. No. 07/569,808, referenced supra. In a third embodiment, relative phase control between successive ROS units, each using encoded signals representing the photoreceptor velocity as the reference signal to be tracked, is introduced to eliminate sub pixel errors in SOS line formation.
More particularly, the present invention relates to an imaging system for forming multiple image exposure frames on a photoconductive member moving in a process direction including:
a plurality of Raster Output Scanners (ROS) units, each ROS unit associated with the formation of one of said image exposure frames, each ROS unit forming a plurality of scan lines in a fast scan (transverse) direction across the width of said member, by reflecting modulated beams from the multi-faceted surfaces of a rotating polygon, each polygon driven by a separate drive motor, PA1 means for detecting at least the beginning of the scan line and providing a start of scan (SOS) signal representing such detection, PA1 means for generating a stable reference frequency output signal, PA1 means for comparing the phase and frequency of said reference output signal with that of the SOS signal, and means for generating an error output signal when a difference in phase and frequency exists, and PA1 means for pulse width modulating drive signals to said polygon motors in response to said error output signal thereby changing polygon speed whereby the phase of the SOS signal is continually adjusted in a feedback loop until said comparing means error output signal is reduced to zero.